1. Field of the Invention
The field of the present invention is phased array antenna systems and, more particularly, modular-type feed systems.
2. Description of the Prior Art
A typical pencil beam phased array radar antenna may contain 4,900 radiating elements and phase shifters. The usual corporate-type sum/difference feed system for this array would require 70 cables and 140 connectors per row of 70 elements plus additional cables and couplers to form the row sum and difference feeds. Thus, more than 2,450 Magic T devices, 4,900 cables and 9,800 connectors are required.
A typical dual corporate feed for providing independent sum and difference beams in a linear array is shown in FIG. 1. Element pairs which are symmetrically located about the center line of the array are connected to the respective side arms of a Magic T device. The sum terminals [S.sub.1 . . . S.sub.m . . . S.sub.M ] of the M Magic T devices are interconnected through another corporate feed to form the sum distribution which is symmetrical (as a consequence of using the sum terminal of the Magic T, but otherwise arbitrary. A typical sum forming network for connection to the sum terminals S.sub.1 . . . S.sub.4 of the Magic T devices is shown in FIG. 2 for M=4.
Since the Magic T devices (T.sub.1 . . . T.sub.M-1) in the sum forming network have arbitrary coupling values, the aperture distribution at the radiating elements is arbitrary except for symmetry about the center line. A network similar to that of FIG. 2, but with another arbitrary set of Magic T devices, is used to connect the difference terminals [D.sub.1 . . . D.sub.n . . . D.sub.M-1 ] in FIG. 1. Since the radiating elements fed by the device are difference terminals, the aperture distribution is arbitrary except for antisymmetry about the center line.
The sum and difference distributions are independent in this known approach, and typical forms are shown in FIG. 3. The ideal Taylor 40 dB sum pattern for a 64-element linear array whose elements are spaced apart by one-half wavelength distances is shown in FIG. 4.
This known corporate feed arrangement is an ideal network for realizing low sidelobes for both the sum and difference patterns because it allows the usage of the maximum number of degrees of freedom available, i.e., M-1 degrees of freedom for the sum pattern and M-1 degrees of freedom for the difference pattern in an array of 2M elements. This network, however, is not ideal from an implementation point of view. Each element requires an interconnecting cable and two connectors. Large cables are preferred to reduce ohmic loss; consequently, a very complex packaging problem must be solved. The cost of cable rawstock, connectors, bending and threading the cables through the antenna array support structure, cutting to electrical length and testing becomes oppressive for large phased arrays. Thus, while this feed admittedly provides the greatest freedom of choice for the sum and difference patterns, the cost and complexity often are prohibitive.
A simpler, prior art technique consists of feeding the radiating elements together in groups to form larger output modules instead of independently feeding each element as in FIG. 1. The modules or subarrays are assumed to be identical and to contain N uniformly illuminated elements. A typical pair of modules symmetrically located about the center of the array are connected together by a Magic T device as shown in FIG. 5. As before, terminals S.sub.m are interconnected to form the sum pattern, and terminals D.sub.m are interconnected to form the independent difference pattern. The principal advantage of this modular approach is that there are far fewer cable connectors and subassemblies.
Although the sum and difference distributions are independent, the number of degrees of freedom has been reduced to M/N-1 for the sum pattern and M/N-1 for the difference pattern (if N=1, the modular feed is the same as the ideal case shown in FIG. 1). Typical aperture amplitude distributions are stepped or quantized as shown in FIG. 6, where the amplitude is constant across a group of N contiguous elements. If these steps are fitted to the ideal Taylor distribution illustrated in FIG. 4, the resultant pattern for a 64-element modular array with 8 elements per module will be as shown in FIG. 7. The sidelobe level has been seriously degraded from -40 dB to -24 dB for the close-in lobe and the remote sidelobes also remain high at typically -34 dB. High sidelobes generally occur near "grating lobe" positions sine .theta.=l.lambda./d, where l=.+-.1, etc., and d is the subarray length. These grating lobe positions are caused by the discontinuities or steps in the aperture distribution resulting from the modular feed, and are analogous to lobe positioning caused by a grating. Thus, the usual equi-amplitude modular array is simpler and more economical than the ideal feed, but the performance degradation often is unacceptable.
It is therefore a principal object of the present invention to provide an array feed system which greatly reduces cost and complexity, yet sacrifices little in sidelobe performance of the resultant radiation patterns.
Yet another object of the invention is to provide a modular feed system adapted to approximate both the desired amplitude and desired slope at the module output.
It is yet another object to provide a modular feed system whose component costs are minimized as a result of substantial use of identical components.
A further object of the invention is to provide a modular feed system whose testing cost is reduced because identical modules may be used which require testing only at the modular level.
It is another object of the invention to provide a modular feed system wherein the number of cables and connectors are reduced, thereby reducing parts costs, as well as facilitating assembly of the feed system into the array backup structure.